A number of flexible packing materials are currently used for packing foodstuffs, pharmaceuticals, medical appliances, and the like. Representative examples of conventional packing materials include single ply films prepared by subjecting a polyolefin resin, such as polyethylene, polypropylene, ethylene-.alpha.-olefin copolymer, ethylene-vinyl acetate copolymer or ethylene-acrylic acid copolymer, to inflation molding, T-die cast molding or the like. Single ply films of polyethylene terephthalate resin polybutylene terephthalate, polyamide resins, and polyacrylonitrile resin, have also been used in the past. In addition, composite films prepared by laminating the above-described films on an appropriate base film or combining the above-described resins to form a film have also been used previously as packaging materials.
Polyolefin resin films have generally been used where heat-sealing capabilities are desired due to the good heat-sealing properties that polylefin films exhibit. However, as compared to films of polyester resins, such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), and polyamide resins, polyolefin films have several physical disadvantages such as lower heat-resistance temperatures, and inferior chemical resistance and gas barrier properties. For these reasons, films of polyester resins have been studied as an alternative to polyolefin films as a heat-sealing layer of packing materials for foodstuffs and pharmaceuticals.
However, in order to improve the heat resistance properties of films formed of PET resins, the films should be subjected to biaxial orientation. Unfortunately, when PET resin films are biaxially oriented, they cannot be heat-sealed. On the other hand, conventional PBT resin films exhibit excellent characteristics when used as a packing material in the absence of biaxial orientation because of their excellent heat resistance, chemical resistance, and gas barrier resistance properties. However, the heat-sealing properties of conventional PBT films is still unsatisfactory. It is therefore towards providing films of PBT resin with improved heat-sealing properties that the present invention is directed.
Broadly, the present invention is directed towards heat-sealable films of polybutylene terephthalate resin that may be usable as a packing material. More specifically, the present invention is embodied in a film formed of a PBT resin composition that includes a PBT base resin, and a heat-sealing effective amount of a polycarbonate resin.
Preferably, the present invention is embodied in a film or a sheet having excellent heat-sealing properties and which comprises a polybutylene terephthalate resin composition composed of a blend of between 50 to 95% by weight of a polybutylene terephthalate resin having an intrinsic viscosity of 0.7 or greater as determined at 25.degree. C. in o-chlorophenol, and between 5 to 50% by weight of a polycarbonate resin having an intrinsic viscosity of 0.4 or greater as determined at 20.degree. C. in methylene chloride.
Further aspects of this invention will become more clear after consideration is given to the following detailed description of the preferred exemplary embodiments thereof.